GENEVA- U.N. officials said Tuesday there has been a sudden fall in the number of Syrian refugees arriving in Jordan, amid reports that Jordan’s government has clamped down on the relentless influx of people escaping Syria’s civil war.

Panos Moumtzis, the top U.N. refugee official for Syria, said the usual flow of up to about 3,000 refugees daily had “dropped to almost zero” over the past four days.

Moumtzis told reporters Tuesday in Geneva that his agency was asking if “people are not being allowed to move, is it security or something else?”

He said the refugee agency lacks staff on the other side of the border and thus was unable to observe the situation there.

Jordanian officials could not immediately be reached for comment on the U.N. concerns, but the Reuters news agency reported Tuesday that, according to multiple sources, Jordan’s government had actively shut down border crossings from rebel-held areas of Southern Syria.

He said at least 1,000 refugees were stranded, “gathering on the border trying to enter Jordan and waiting for the border to open.”

According to the Reuters report, which cited diplomats, aid workers and refugees, the four unofficial crossing points into Jordan from Syria were effectively closed six days ago, and only the one official crossing at Jaber remained open.

A diplomat, who was not named, told Reuters the flow of refugees was being halted over security concerns inside Jordan, where the government has come under increasing pressure to reduce the burden placed on the state from the roughly half a million Syrian refugees now in the country.

“The Jordanians are worried about security issues (and) are sending signals to the international community highlighting the huge refugee burden they are now shouldering,” Reuters quoted the diplomat as saying.

Conditions at the swollen, primary refugee camp inside Jordan — the Zaatari camp — have become so bad that riots have broken out over safety concerns and access to basic goods.